The present invention relates to a light diffusing film and a method of producing the same as well as a screen which is formed using the light diffusing film.
In recent years, an overhead projector or a slide projector is used widely as an apparatus which is used by a speaker in a meeting or the like to present information materials. Also in ordinary homes, a video projector or a moving picture film projector which is formed using liquid crystal is being popularized. Such projectors as mentioned above employ a projecting method wherein light outputted from a light source is optically modulated, for example, by means of a liquid crystal panel of the transmission type or the like to form image light, and the image light is emitted through an optical system such as a lens so as to be projected on a screen.
For example, a projector apparatus which can form a color image on a screen includes an illuminating optical system for demultiplexing a light beam emitted from a light source into color light fluxes of red (R), green (G) and blue (B) and converging the color light fluxes into predetermined light paths. The projector apparatus further includes a liquid crystal panel (light valve apparatus) for optically modulating the color light fluxes of R, G and B demultiplexed by the illuminating optical system. The projector apparatus further includes a light multiplexing section for multiplexing the color light fluxes of R, G and B optically modulated by the liquid crystal panel. Thus, a color image multiplexed by the light multiplexing section is projected in an enlarged scale on a screen by a projection lens.
Recently, also a projector apparatus of a type wherein a narrow-band three-primary color light source is used as a light source and a grating light valve (GLV) apparatus is used in place of a liquid crystal panel to spatially modulate color light fluxes of R, G and B has been developed.
In the projector apparatus just described, a projector screen is used in order to allow a viewer to watch a projected image. Projector screens can be roughly divided into a front projector screen on which projection light is illuminated from the front side such that the view watches the projected light reflected from the screen, and a rear projector screen on which projection light is illuminated from the rear side such that the viewer watches the light transmitted through the screen from the front side of the screen. In recent years, while the size of the screen has been increasing with the enhancement of the performance of the projector, it is demanded for a screen of any type to have a characteristic that the visibility is good with a high luminance free from non-uniformity.
Of whichever type a screen is, generally a light diffusing film for diffusing light is provided on the surface of the screen. Since image light is diffused by and emitted uniformly from the light diffusing film, the viewer can visually sense the projected image. Accordingly, in order to implement a screen having the characteristic described above, special functions are demanded for the light diffusing film.
A first one of the functions required for the light diffusing film is that, in order to utilize image light effectively to assure a high luminance of the screen, light can be diffused and emitted restrictively toward a region in which presence of an observer is estimated. A second one of the functions is that the luminance of a peripheral portion of the screen which is unfavorable in regard to the visual angle when compared with a central portion of the screen is enhanced to reduce the non-uniformity in luminance.
The first function distributes image light only toward a region in which it is estimated that a viewer or viewers are present. If it is supposed that the seated positions of a viewer is fixed, then considerable diffusion is not required in the upward and downward directions of the line of sight (in the vertical direction on the screen). However, in regard to the horizontal direction, preferably the diffusion is great to some degree from the point of view of the range of the position at which the viewer is seated in an opposing relationship to the screen (width of a row of viewers). Where unnecessary diffusion in the vertical direction is eliminated in this manner, the luminance of the entire screen is enhanced and a projected image which is less liable to be influenced by external light is achieved. This function is generally called anisotropic diffusion. In this instance, it is necessary for anisotropic diffusion to be small in the vertical direction but greater in the horizontal direction.
Light from a projector projected on a screen is diffused on the surface of the screen with respect to an optical axis directed in the direction of regular reflection. Accordingly, most of light diffused by and emitted from an upper end or a lower end of the screen is radiated uselessly to the outside of the screen. Particularly where the anisotropic diffusion is low in the vertical direction, the flux of light toward the viewer further decreases. In particular, since a region in which the luminance is low appears at the upper end or the lower end of the screen, the region is perceived as a dark striped portion by the viewer and obstructs good appreciation. This is luminance non-uniformity, and the second function eliminates or moderates the luminance non-uniformity.
Meanwhile, a transparent film formed by dispersedly applying resin particles to a resin binder, a resin film having a surface worked into a matted state and a like film are conventionally available. However, such conventional films as just mentioned isotropically diffuse projected light from a projector and cannot avoid lowering the luminance. Simultaneously, it is very difficult in principle for the films mentioned to control the anisotropic diffusion of light.
On the other hand, in order to implement anisotropic diffusion of light, a method is available wherein a speckle pattern appearing when a coherent light flux is illuminated on a roughened surface is formed on a photosensitive material. The method is disclosed, for example, in Japanese Patent No. 3,413,519. The speckle diffuser is advantageous and very useful not only in that effective anisotropic diffusion of light can be obtained regarding individual speckle patterns as micro lenses having a major axis in the vertical direction but also in that, since the speckle patterns are arranged at random in configuration, a moire pattern or coloring by interference of light does not appear.
However, if the diffuser disclosed in Japanese Patent No. 3,413,519 is used for a screen of the reflection type, then a portion at which the luminance is insufficient appears at an upper end portion and a lower end portion of the screen because the diffusion in the vertical direction of the field of view is small, and this tendency appears comparatively conspicuously as the screen size increases. In this instance, it is possible to enhance the luminance against insufficiency by forming an upper end and a lower end of the screen in a curved configuration or by disposing the screen so as to be inclined toward the viewer. However, the countermeasures are not suitable for universal use because they are restricted in practical use in that it is difficult to take up and accommodate the former while the latter requires provision of a special inclining mechanism.
Therefore, a screen wherein a light transmitting diffusing layer is provided on the front surface side of a transparent sheet and a linear Fresnel lens for reflecting light is provided on the rear surface side of the transparent sheen has been proposed and is disclosed in Japanese Patent No. 3,341,225. In the screen of Japanese Patent No. 3,341,225, the Fresnel lens surface formed on the rear surface side of the transparent sheet not only reflects projected light but also has an action as an aspheric concave mirror for condensing light. Therefore, the screen has a performance of converging and reflecting light efficiently toward the viewer side.
However, according to the screen disclosed in Japanese Patent No. 3,341,225, not only the front surface side but also the rear surface side of a transparent sheet must be worked. This cannot avoid increase of the expenses for the equipment and decrease of the yield of products, resulting in increase of the cost.